Once all of your preparations are complete, head toward the amphitheater to meet the appropriate character. Just who you'll meet depends on which quest you played previously. If you went with Where is Triss Merigold?, then she is the one you meet. Otherwise, you'll meet Iorveth or Roche, depending on who your ally is for this last leg of the adventure.

If you are with Triss, you'll possibly have to beat some guards in order to proceed. Otherwise, simply head into the summit without resistance. Inside, a lengthy scene will follow that doesn't require your active participation. Just watch everything unfold and enjoy the plot twists. Then when the mission concludes, the Enter the Dragon story quest is activated.